1. Field of the Invention
This invention relates to composite fabrics and to a method of making composite fabrics, and more particularly, to an improved method of adhering or bonding the layers of composite fabrics made of fibrous material to provide an improved composite preform.
2. Description of the Prior Art
Composite fabrics made from fibrous materials formed into both woven and non-woven material, are well-known in the art. Yarns of glass, carbon and graphite are typically woven into fabrics, and a plurality of layers of fabric are stacked and cut into dry fabric preforms. The preforms are then stitched and/or impregnated with a resin binder to form a rigid composite fabric.
Many composite fabrics made from fibrous materials, such as yarns, fibers, filaments and the like, have substantial rigidity and other properties which make them desirable for many applications. Certain of the composite fabrics even have sufficient rigidity and strength so that they can be formed into components heretofore made from metals. For example, many composite fabrics have sufficient strength, rigidity, impact resistance and other desirable properties which make them useful materials for turbomachinery blading members, for example fan blades for jet engines. However, in making composite fabric preforms of fibrous materials, it is difficult to hold mechanically the layers of substantially dry structural fabrics and to prevent damage to the yarns, fibers and/or filaments in the stacked assembly of such dry fabric layers during various operations such as cutting, stitching and the like.
Fiber impregnation with plastics materials is well-known in the prior art, and in U.S. Pat. No. 4,131,502, unidirectional fibers and auxiliary strips of non-fibrous or fibrous materials are impregnated with plastics materials. This is accomplished by placing sheets of plastic material upon release sheets, applying the unidirectional fibers and auxiliary fibrous or non-fibrous material to the surface of the sheet of plastics material, applying a second release sheet upon the surface of the unidirectional fibers and auxiliary fibrous and non-fibrous material. Thereafter heat and pressure are applied such that the plastic sheet material flows about the unidirectional fibers to form a series of coherent ribbons. This produces ribbons suitable for use in filament windings, and the plastics material serves as a reinforcing material to the ribbons and to the filament windings made therefrom. In U.S. Pat. No. 4,131,502, the plastics materials may comprise a single thermosettable resin, such as an epoxide resin and other conventional thermosettable resins.
Rigid, perforated cloth is prepared in the prior art by wrapping a core of yarn with a covering yarn made from a thermoplastic polymer to produce a completely wrapped yarn. The yarn then is woven into a perforated cloth in both the warp and weft directions. In U.S. Pat. No. 4,126,499, the foregoing cloth made from the wrapped yarn is heated to a temperature in excess of the melting temperature of the thermoplastic covering polymer, but below the deformation and melting temperatures of the core yarn, in order to melt the covering thermoplastic polymer. The heated cloth is then cooled to provide a rigid, perforated cloth in which the points of intersection of the wrapped yarn adhere to each other due to the prior melting of the thermoplastic covering polymer.
In U.S. Pat. No. 4,144,115, a tubular sheath is made by weaving inorganic fiber threads having a plurality of spaced axial threads which form crossing points with the threads of the braid or weave, the axial threads comprising a mixture of at least two thermoplastic fibers having different softening temperatures. The tensioned braid or weave is heated to a temperature sufficient to soften the thermoplastic fiber having the lower softening temperature to bond the latter to the inorganic fibers at their crossing points only, and weakening the thermoplastic fibers having the higher softening temperature to enable the latter to be only slightly elongated during tensioning. This is intended to substantially equalize stresses within the thermoplastic fibers having the higher softening temperature so that the latter are of the same length following release of the tensioning, cooling the sheath and releasing the tensioning.
Non-woven fabrics, having an upper and a lower surface and comprising dyeable, flame-resistant fibers and base fibers are disclosed in U.S. Pat. No. 4,194,037. There the fabric is formed by needling base fibers together to form a batt, needling polyvinyl chloride fibers and the batt together, and then heating the lower surface made of a blend of adhering base fibers and polyvinyl chloride fibers to adhere the polyvinyl chloride and base fibers together.
Although the various prior art references teach the melting of heat softenable materials distributed therein to promote adherence or a bonding action between various fibers, the prior art references are directed to the joining of individual strands to each other or to a backing sheet. However, as discussed above, when composite fabrics made of fibrous materials are placed in stacked layers or plies so that the layers are adjacent each other, for the purpose of forming dry fabric preforms, they are difficult to handle. It is difficult to carry out subsequent operations on such stacked assemblies of fabrics. For example, it is difficult to cut, to stitch and impregnate the stacked fabric assemblies with resins. Mere mechanical means for holding the dry structural composites of fibrous materials, for example, of glass and graphite, are inadequate because even though the layers of fabric can be mechanically secured by various mechanical means such as, clamping devices, pressure platens and vacuum manifolds, the fabrics become frayed during initial ply cutting operations and various other operations which damage the fibers. Furthermore, in many cases when resins are impregnated into the composite fabrics, and the mechanical holding means is removed therefrom for the purpose of stitching the composite fabrics in a transply or other direction, the needling operation severs and fragments yarns, threads, fibers and filaments which are securely held in the cured resin material.